This invention relates to insulating foams, and more particularly to foams having unusually low thermal conductivity.
Insulating foam, particularly polyurethane foam, is an indispensable constituent of many refrigerators and similar cooling units. By reason of increasingly strict Federal Government energy conservation standards, it is of interest to develop foams having substantially lower thermal conductivity than those presently available.
Conventional insulating polyurethane foam is generally prepared by the reaction of at least one polyol with at least one diisocyanate in the presence of suitable catalysts, surfactants and blowing agents. Among the blowing agents employed are halogenated compounds, which may be halocarbons such as monofluorotrichloromethane and/or halohydrocarbons such as 1,1-dichloro-1-fluoroethane. They remain in the cell voids of the foam after blowing, and their presence therein is beneficial since they have low thermal conductivity and thus contribute to the insulating quality of the foam.
Also usually present in the reaction mixture is water, which serves at least three purposes. First, in small quantities it is effective to improve the flow properties of the mixture. Second, it reacts with a minor proportion of the diisocyanate to form carbon dioxide, which is also effective as a blowing agent. Third, it reacts to form substituted urea crosslinking moieties, thus stabilizing the foam as it is produced.
The use of many halocarbons and halohydrocarbons is now severely restricted or, in some cases, prohibited by reason of their properties of depleting the stratospheric ozone layer and promoting greenhouse global warming. It is therefore necessary to develop methods for blowing insulating foam which do not require the use of halogen-containing blowing agents. At the same time, however, a high degree of insulating power is more necessary than ever to decrease energy consumption. An additional desirable factor is suppression of the "greenhouse effect" which promotes global warming.
Carbon dioxide has a high thermal conductivity, on the order of 17 milliwatts/m-.degree.K. Thus, a foam prepared with the use of a blowing agent combination including 15% and 50% (by volume) carbon dioxide with the balance being monofluorotrichloromethane has a thermal conductivity approximately 5% and 15%, respectively, higher than a corresponding foam prepared without the use of carbon dioxide.
In U.S. Pat. No. 5,371,113, there is described a method for preparing polyurethane foam of low thermal conductivity by incorporating in the foam, at the time of its preparation, a solid reagent or "getter" which is incorporated in said foam and which is capable of removing said carbon dioxide. Alkaline materials are particularly useful for this purpose, with sodium hydroxide often being preferred. U.S. Pat. No. 5,389,695 describes a similar method in which the foam is sealed in a gas-impervious enclosure in the presence of an inert gas of low thermal conductivity, such as krypton or xenon, at a partial pressure in the range of about 20-200 torr. The disclosures of both of said patents are incorporated by reference herein.
The reaction of sodium hydroxide with carbon dioxide produces sodium carbonate and water, each being formed in an equimolar amount with respect to the carbon dioxide which has reacted. Water, however, also has high thermal conductivity. In the vapor state, its conductivity is essentially the same as that of carbon dioxide. In addition, any liquid water present is an efficient conductor of heat.
It might be thought that the inclusion of a drying agent in the system could result in removal of whatever water is formed, further reducing the thermal conductivity of the system. However, the reaction of getters such as anhydrous sodium hydroxide with carbon dioxide in the presence of most drying agents is so slow as to be commercially unpracticable. The presence of minor amounts of water in the system is necessary for the reaction to proceed at a suitable rate.